Ophthalmic microscopes are essential instruments that can be found in the rooms of every ophthalmologist and optometrist. The most common models are those made by Haag Streit and Zeiss. Although other manufacturers exist, such as CSO, their microscopes generally follow the design of one of these two producers.
All ophthalmic microscopes have a number of common elements in their design. They all consist of a base with a support frame for positioning the eye of a patient. The frame has a chin rest and a forehead strap that together position the patient's head in a central position that allows easy viewing of the eye. A slit lamp and binocular microscope are mounted on a swivel base that allows adjustment of the apparatus to focus on either the left or right eye.
Depending on the manufacturer and model, the slit lamp may be mounted above, below or beside the binocular head. The slit lamp is adjustable to focus a slit of light on the retina of a patient. The focus of the binocular microscope may also be adjusted to focus on the part of the eye requiring attention.
Whatever the optic configuration, it is essential that the optical components are correctly aligned. The optical accuracy required makes these instruments expensive and delicate.
Lasers have found application in treatment of the eye. In the early days of laser eye treatments the lasers were relatively large and cumbersome. However, recent advances in technology have resulted in very compact lasers with relatively high power. It is now possible to produce a laser with sufficient power for many eye treatments that is small enough to mount on a slit lamp microscope. A number of different mounting arrangements have been proposed.
In one scheme, the laser is mounted beneath the base of the microscope with the beam directed upwards. The beam is steered by prisms to a partial mirror in front of the slit lamp microscope and light tower. This arrangement requires separate alignment and focusing of the laser beam to the optic axis of the slit lamp. A variation of this design has the prisms integrated to the light tower so that the laser beam moves with the slit lamp beam.
In a few prior art arrangements, small, lightweight lasers have been mounted directly on the slit lamp light tower. These designs are functionally similar to the above design but with the advantage of less optical elements being required. However, the arrangement compromises the independence of the illumination angle of the light source which can result in reduced visibility. For example, to optimally illuminate an ocular structure it is frequently necessary to illuminate non-coaxially. As the angle increases, the aperture of the illumination source is increased by the human iris. Aperturing of the slit lamp image is quite acceptable but aperture of the laser beam results in beam clipping which is undesirable. It is preferable to lock the laser axis to the viewing axis and leave the illumination axis unconstrained.
In another design, the laser is mounted above, below or beside the binocular microscope with the beam directed to a partial mirror before the objective lens of the microscope. This arrangement has the advantage of aligning the optic axis of the laser with the view of the ophthalmologist. However, if the alignment is truly coaxial, the slit lamp head will block the laser beam. This design therefore has the problem of the optic axis of the slit lamp and the laser beam being slightly displaced.
It will be appreciated that each of the generic designs described above require permanent attachment of the laser to the slit lamp in order to achieve and maintain correct optic alignment. This requires a substantial financial investment for the ophthalmologist since each slit lamp must have a dedicated laser. It is typical for an ophthalmologist to visit a number of surgeries, so multiple laser units may be required. The required investment is beyond the reach of many practitioners so that laser ophthalmology is not as common as it could be.